N-doped ordered mesoporous carbon spheres derived by confined pyrolysis for high supercapacitor performance

Accepted Manuscript Title: N-doped ordered mesoporous carbon spheres derived by confined pyrolysis for high supercapacitor performance Authors: Juan Du, Lei Liu, Yifeng Yu, Yue Zhang, Haijun Lv, Aibing Chen PII: DOI: Reference:

S1005-0302(19)30153-7 https://doi.org/10.1016/j.jmst.2019.05.029 JMST 1576

To appear in: Received date: Revised date: Accepted date:

26 March 2019 18 April 2019 30 April 2019

Please cite this article as: Du J, Liu L, Yu Y, Zhang Y, Lv H, Chen A, Ndoped ordered mesoporous carbon spheres derived by confined pyrolysis for high supercapacitor performance, Journal of Materials Science and amp; Technology (2019), https://doi.org/10.1016/j.jmst.2019.05.029 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Research Article N-doped ordered mesoporous carbon spheres derived by confined pyrolysis for high supercapacitor performance

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Juan Du, Lei Liu, Yifeng Yu, Yue Zhang, Haijun Lv, Aibing Chen *

Science and Technology, Shijiazhuang 050018, China

*

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College of Chemical and Pharmaceutical Engineering, Hebei University of

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Corresponding author: E-mail address: [email protected] (Aibing Chen).

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[Received 26 March 2019; Received in revised form 18 April 2019; Accepted 30

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April 2019]

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Herein, we report a confined pyrolysis strategy to prepare mesoporous

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carbon nanospheres by which surface area of carbon spheres is increased, pore size is enlarged and effective N-doping is achieved. In this method, the mesoporous polymer sphere as carbon precursor and 2-methylimidazole as

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nitrogen precursor are encapsulated in a compact silica shell which provides

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a confined nano-space for the pyrolysis treatment. The in situ generated gases from mesoporous polymer sphere and 2-methylimidazole under pyrolysis

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diffuse into the pores of mesoporous polymer sphere in the confined compact silica shell, resulting in increased surface area, larger pore size and N-doping due to self-activation effect. As electrodes in supercapacitor, the N-doped mesoporous carbon nanospheres exhibit a significantly enhanced specific capacitance of 326 F g-1 at 0.5 A g-1, which is 2 times higher than that of

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mesoporous carbon spheres under unconfined pyrolysis condition, exhibiting its potential for electrode materials with high performance.

Keywords: Confined pyrolysis; N-doping; Ordered large mesoporous spheres;

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Supercapacitor 1. Introduction

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As the new energy storage system, supercapacitors have significant potential

due to their high power density, long cycle life, and fast charge/discharge rates, etc. . In recent years, due to their distinctive electrical properties, supercapacitors

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[1-4]

. The properties of electrode materials including surface area, pore size and pore

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7]

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have been extensively used in portable electric transports and electronic devices [5-

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volume absolutely determine the energy-storage ability of supercapacitors based on electric double-layer capacitors (EDLC) mechanism [8-10]. The ordered mesoporous

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carbon spheres (MCS) can provide large inner surface area for storing charges and favor electrolyte ion diffusion kinetic process in the electrodes, therefore improve

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the electrochemical performance [11]. In recent years, many studies have been carried out on design and application of MCS for supercapacitors [12, 13]. A main

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goal of these studies is to increase specific area and adjust pore size so as to improve the electrochemical performance for MCS [14, 15]. In addition, nitrogen

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functional groups not only enhance the surface wettability of electrodes but also significantly increase the specific capacitance by the Faradic reaction occurring during the electrochemical charge-discharge process [16]. Therefore, achieving nitrogen doping is also one of the key ways to effectively improve electrochemical performance.

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Large surface area in MCS provides large space for ion accumulation, enhancing the electrochemical features of MCS. To increase the surface area and pore volume, post chemical and physical activation processes have been developed [17-20]

. Chemical activation treatment by KOH, ZnCl2, or physical activation

treatment by CO2 can increase the surface area and pore volume [21, 22]. However, it

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should be noted that the activation treatment takes effect only under rigorous

activation conditions. Meanwhile, it also will lead to the collapse of mesopores and

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deteriorated order degree, limiting the efficient transport of charge and electrochemical performance.

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As investigated in many studies, large mesoporous size can shorten the ions

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diffusion distance and reduce the diffusion resistance to obtain high

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electrochemical performance [14, 23]. Traditionally, mesoporous size is controlled by

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the template in template method. Jerzy et al. used silica primary oligomer particle

[24]

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as hard template to create large mesopores and the mesopore size reached 13.5 nm . However, the mesoporous structure was disordered which limited its further

modification and wide application. Thermally decomposable amphiphilic

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molecules as hard template or organic-organic self-assembly of thermosetting carbon precursors were also used to generate large pore size. For example, large

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mesoporous size (up to 16 nm) can be created using high-molecular-weight block

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polymer PS-b-PEO micelles as template [25]. But it's not easy to control the micelles structure. Another solution to increase the total electrical energy storage capacitance of supercapacitors is based on fast redox reactions happening at the electrode surface, resulting in pseudo-capacitance [26]. The surface properties of the MCS are vital for

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pseudo-capacitors behavior. Heteroatoms, including nitrogen, sulfur and phosphorus, have been introduced into carbon to modify the surface and frame properties [11]. Nitrogen is especially attractive because of its smaller atomic radii and higher electronegativity than that of carbon

[27, 28]

. The doped active pyridinic

and pyrrolic nitrogen (along with quinone oxygen groups) give rise to pseudo-

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capacitance [29, 30].

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Herein, we reported a confined pyrolysis strategy to prepare monodispersed N-doping ordered mesoporous carbon spheres (N-MCS), realizing increased

surface area, enlarged mesoporous size and N-doping synchronously. The ordered

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mesoporous polymer sphere (MPS) was employed as carbon precursor. 2-

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Methylimidazole (Hmim) was adsorbed into MPS and used as nitrogen precursor.

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At the same time, Hmim acted as catalysis for formation of compact silica shell

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outside MPS. Consequently, the compact silica shell built an encapsulated

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nanoreactor for the pyrolysis of MPS and Hmim. During the pyrolysis process, the gas released, which increased the surface area and pore size of resultant MCS via self-activation effect. In addition, Hmim decomposed into nitrogen-containing gas

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which led to nitrogen doping in carbon framework. The resulted N-MCS had high surface area, large mesoporous size and a certain of nitrogen content, which

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allowed for faster charge transport and more ion accumulation, leading to better

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electrochemical performance. 2. Experimental 2.1. Preparation of N-MCS The order mesoporous polymer was prepared by previous research

[31]

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Typically, phenol (0.6 g), formalin aqueous solution (2.1 mL, 37 wt%) and NaOH

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aqueous solution (15 mL, 0.1 mol L-1) were mixed and stirred at 70 °C for 0.5 h to obtain low-molecular-weight phenolic resoles. After that, 0.96 g of triblock copolymer Pluronic F127 (average molecular weight Mw = 12600, PEO-106-PPO70-PEO-106) dissolved in 15 mL of H 2O was added. Then the mixture was stirred at 70 °C for 2-4 h. After that, 50 mL of water was added to dilute the solution. The

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reaction was stopped when the deposit was observed. After quiescence until the

deposit was dissolved, 17.7 mL of the obtained solution was diluted with 56 mL of

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H2O and transferred into an autoclave and heated at 130 °C for 24 h. The MPS

nanoparticles was collected by centrifugation and washed with water for several

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times.

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To prepare polymer/silica nanohybrids (named as MPS@SiO 2) nanoparticles

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with encapsulated nitrogen precursor, as-prepared MPS nanoparticles (124 mg)

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were dispersed into 100 mL of ethanol solution with 0.62 g Hmim (mass ratio of Hmim and MPS wHmim : wMPS= 5) for stirring 12 h to obtain composites of MPS and

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Hmim (MPS/Hmim). Then, 1 mL of TEOS was then introduced into the above solution accompanying by stirring and reacted at room temperature for overnight.

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The product of MPS@SiO2 nanoparticles was collected using centrifugation (9500 rpm) and rinsed several times with ethanol and dried at 50 °C for 10 h. The as-

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prepared MPS@SiO2 nanoparticles were heated at 2 °C min -1 from room temperature to 800 °C and kept at this temperature for 3 h under a nitrogen flow.

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The pyrolysis product was treated with aqueous HF solution (10 wt%) to remove the silica and generate N-MCS. Under the same reaction conditions for synthesis of MPS@SiO2 core-shell spheres, the nitrogen precursor was changed to Ethanediamine (EDA), Thiourea and Pyridine respectively (0.248 g) in the synthesis of N-MCS-EDA, N-MCS-Thiourea and N-MCS-Pyridine, respectively. In

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order to investigate the effect of silica shell, the MCS was also prepared by same process with N-MCS without silica shell. 2.2. Characterization X-ray diffraction (XRD) patterns were achieved using a Rigaku D/MAX-2500 system with CuKα (15406 nm). The morphology and microstructure of samples

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were investigated by transmission electron microscopy (TEM, JEOL JEM-2100) and scanning electron microscopy (SEM, HITACHI S-4800-I). Nitrogen

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adsorption-desorption isotherms were carried out on a Micromeritics TriStar 3020 instrument at -196 °C. The Brunauer-Emmett-Teller (BET) method was employed

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to calculate the specific surface area, while the Barrett-Joyner-Halenda (BJH)

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method was applied to analyze the pore size distribution using the desorption

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branch of isotherm. The total pore volume was obtained from the amount of N 2

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adsorbed at the relative pressure (P/P0=0.97). Thermogravimetric analysis (Pyris 1 TGA) was performed under air flow from 20 to 800 °C at a heating rate of 10 °C

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min-1. X-ray photoelectron spectroscopy (XPS) was conducted on a Thermo Scientific ESCALab 250Xi system using an Al-Kα radiation under a vacuum of

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3×10-10 mbar.

2.3. Electrochemical Measurements

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The working electrode was prepared by coating the viscous slurry (samples, carbon black and polytetrafluoroethylene with the mass ratio of 8:1:1 in ethanol)

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onto Ni foam current collector. The mass of active material loaded on each working electrode was 4-5 mg after drying at 100 °C for 24 h. Electrochemical measurements were carried out in both three-electrode and two-electrode using an electrochemical workstation (CHI 760E, Chenhua Instruments, China) with 6 mol L-1 KOH solution as the electrolyte. Electrochemical performances were evaluated

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by cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrical impedance spectroscopyanalysis. For three-electrode system, a Pt wire and Hg/HgO was used as the counter and reference electrodes. For the fabrication of supercapacitor devices, two slices of electrode were immersed in 6 mol L-1 KOH and were separated by a filtration paper, then tested by the current collector.

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Electrochemical performances were evaluated by GCD. In three-electrode system, the specific gravimetric capacitance was calculated according to the GCD

C=I∆t/∆Vm

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measurements: (1)

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where I (A), ∆t (s), ∆V (V) and m (g) are GCD current, discharge time, voltage

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window, and mass of active material, respectively. For the two-electrode system,

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the specific capacitances (C, F g-1), energy density (E, W h kg-1) and power density

E=0.5C(∆V)2 P=E/∆t

(2) (3)

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C=4I∆t/∆Vm

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(P, W kg-1) were calculated by the following equations:

(4)

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3. Results and discussion

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Synthesis of N-MCS by confined pyrolysis strategy was illustrated in Fig. 1(a). Firstly, Pluronic F127 and phenol/formaldehyde were used as a template and

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carbon precursor to synthesize MPS according to previous work [31]. The rich porous MPS had good adsorption properties. When Hmim was added, it could be adsorbed into the pores of the MPS, and simultaneously it can catalyze the hydrolysis of the silica source. Thus, a uniform and compact silica layer was coated on the MPS as shown in route 1 of Fig. 1(a), building a confined nano-reactor.

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During pyrolysis process, gas (CO2, H2O, etc.) decomposed from Hmim and MPS would activate carbon wall of MCS [23], increasing the surface area, pore volume and mesoporous size. At the same time, Hmim decomposition also led to in situ Ndoping for carbon skeleton. By contrast, when the MPS impregnated with Hmim was directly carbonized (Fig. 1(a), route 2), ordered mesoporous carbon sphere

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with smaller mesoporous size (denoted as MCS) was obtained as previously

reported [31]. Therefore, the confined condition for the pyrolysis of MPS and

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nitrogen precursor was extremely important to achieve larger mesopores and nitrogen doping. In order to investigate the adsorption of Hmim on MPS,

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MPS/Hmim composite with saturated adsorption was prepared. The TGA analysis

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was used to estimate the content of Hmim in MPS (the blue frame in Fig. 1(a)). It

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was clear that Hmim decomposed completely at high temperatures (200 °C). The

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MPS/Hmim composite showed lower carbon residue (30.2%) than that of MPS (33.7%), indicating the adsorption capacity of MPS for Hmim was approximately

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115 mg g-1. The successful adsorption of Hmim in MPS provided a prerequisite for N-doping in N-MCS.

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Fig. 1(b) and (c) shows the TEM images of polymer/silica nanohybrids (named as MPS@SiO2) with core-shell structure. Clearly, MPS showed particle

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with diameter ca. 100 nm (the red circle) and obvious mesoporous structure. The

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thickness of rough, compact silica layer was 35 nm (the red sign), coating outside MPS core. This compact silica shell provided a confined nano-space for the pyrolysis of MPS and Hmim, which was essential for increasing mesoporous size and effective nitrogen doping. The SEM image (Fig. 1(d)) of N-MCS revealed that the average particle size

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was ca. 100 nm, the same as the diameter of MPS, indicating that no apparent shrinkage of MPS occurred during pyrolysis under confined condition. Decomposition of Hmim led to heteroatom doping to N-MCS. As shown in inset of Fig. 1(d), the X-ray energy dispersive spectrum (EDS) pattern and elemental composition of N-MCS represented C, O and N in N-MCS, demonstrating

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successful N-doping under the confined pyrolysis condition. The ordered

mesoporous structure of N-MCS was obviously seen from the TEM image (Fig.

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1(e)). It was notable that the mesoporous size of 8.0 nm could be observed (inset of Fig. 1(e)), which is larger than that of MCS (approximately 3.0 nm in Fig. 1(f)),

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demonstrating the confined nano-space led to enlarged mesoporous size.

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The different textural properties of the N-MCS and MCS were analyzed by

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nitrogen isothermal adsorption-desorption measurements. As demonstrated in Fig.

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1(g), N-MCS showed a type of IV adsorption-desorption isotherm and H3

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hysteresis loop, indicating mesoporous structure in N-MCS [13]. Fig. 1(h) exhibits the pore size distribution of N-MCS and MCS calculated by adsorption branches. It was clear that N-MCS showed larger pore sizes with 8.0 nm compared with MCS

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(3.0 nm), agreeing well with the TEM results. Table 1 provides the detailed textural parameters of the N-MCS. The N-MCS possessed maximal specific surface area of

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1602 m2 g-1 and pore volume of 2.09 cm3 g-1, which were higher than that of MCS.

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The increased specific surface area and pore volume were attributed to the selfactivation effect under confined pyrolysis conditions. The small angle XRD spectra of the N-MCS and MCS were tested to investigate the order degree of products as shown in Fig. 2(a). The peaks at 1.10° and 1.99° for MCS could be indexed as (110) and (211), which indicated the highly

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ordered structure. However, the diffraction peaks of N-MCS were weaker and wider, demonstrating that the order degree in N-MCS was destroyed to a certain extent due to self-activation and N-doping behavior during the confined pyrolysis process.

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Hmim was used as nitrogen precursor to achieve N-doping for carbon framework. Fig. 2 shows the XPS of N-MCS and MCS. As shown in Fig. 2(b), the

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survey curve of N-MCS and MCS presented three peaks at 284.6, 531.6.0 eV as

well as 399.6 eV, corresponding to C1s, O1s and N1s, respectively. Compared with the MCS (C-96.7%, N-0.6%, O-2.7%), the N-MCS showed obviously higher

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nitrogen content (C-93.2%, N-2.6%, O-4.2%), indicating more nitrogen species

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reserved under the confined nano-space condition. In XPS spectrum of C1s for N-

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MCS and MCS (Fig. 2(c)), the three sub-peaks were 284.6 (graphitic C), 285.7 (C-

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N/C-O) and 287.9 eV (C=O) respectively [32]. The O1s spectrum (Fig. 2(d)) of N-

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MCS and MCS could be divided into three peaks at 532.3, 530.6 and 533.7 eV which corresponded to C=O, C-O and hydroxyl oxygen respectively. However, the proportion of C-O located at 530.6 eV in N-MCS was higher than that of MCS,

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which might be attributed to more O content derived from the role of confined condition. Fig. 2(e) shows the forms of nitrogen in N-MCS and MCS. High-

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resolution XPS N1s spectra for N-MCS and MCS revealed the assignments of three

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types of nitrogen, corresponding to pyridinic-N, pyrrolic-N and graphitic-N, respectively. At the same time, a higher proportion of pyridinic-N located at 398.6 eV in N-MCS could be found, which played an important role for enhanced electrochemical performances. The different nitrogen species incorporated into carbon framework was shown in Fig. 2(f) [33].

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This mechanism of confined pyrolysis is described as Fig. 3. The nitrogencontaining precursor (Hmim) could be adsorbed on the rich mesoporous MPS, which was one of the necessary conditions for N-doping in carbon framework. The composite of MPS and nitrogen precursor was encapsulated in the silica shell to form a confined nano-space as shown in Fig. 3a. During the process of pyrolysis

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(Fig. 3(b)), in situ generated gases (CO 2 and H2O, etc.) from pyrolysis of MPS and nitrogen precursor were confined in the silica shell and furtherly diffused into the

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pores of MPS. CO2 and H2O were excellent activators, which exerted self-

activation effect in etching adjacent pore walls, forming large pores, and also

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increasing surface area [34]. In the meantime, the nitrogen species derived from

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nitrogen precursor were incorporated in the carbon framework, resulting in uniform

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N-doping.

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In fact, this confined pyrolysis strategy is a universal method for preparing N-

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doped mesoporous carbon with enlarged mosopore size and increased surface area. Some other nitrogen-containing substances (including Ethanediamine (EDA), thiourea and pyridine) were also selected as nitrogen precursors and catalysts to

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fabricate N-MCS. From the thermogravimetric analysis (TGA) (Fig. 4(a)), the approximately adsorption amount of EDA, thiourea and pyridine were 373, 112 and

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112 mg g-1 respectively, which provided prerequisites for N-doping in carbon

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framework. After the confined pyrolysis process, the corresponding N-MCS-EDA, N-MCS-Thiourea and N-MCS-Pyridine were obtained. TEM images of the samples (Fig. 4(b)) obviously showed uniform spherical morphology and ordered mesoporous structure, and the mesoporous size was evidently larger than MCS. Nitrogen adsorption-desorption isotherms and pore size distribution profiles of the samples was presented in Fig. 4(c) and (d). The detailed textural parameters of the 11

samples with different nitrogen precursor were shown in the Table 2. Compared with MCS, all the three samples demonstrated greatly increased surface area and enlarged mesoporous size, indicating the effect of self-activation during the confined pyrolysis process. The XPS test was also examined to investigate the chemical state of nitrogen in these samples as shown in Fig. 4(e)-(h). The survey

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curves (Fig. 4(e)) of N-MCS-EDA, N-MCS-Thiourea and N-MCS-Pyridine also presented C1s, O1s and N1s peaks (Fig. 4(f)-(h)). N1s signal demonstrated

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successful N-doping. In Fig. 4(i), it is observed that nitrogen contents of N-MCS-

EDA, N-MCS-Thiourea and N-MCS-Pyridine were higher than that of MCS, which

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was ascribed to the confined environment. Therefore, the confined pyrolysis

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strategy was a universally applicable method to enlarge mesopore size, increase

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surface area and achieve N-doping in N-MCS from different nitrogen precursors.

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Uniform spherical morphology, large surface area, pore volume and

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mesoporous size as well as active nitrogen species were favorable features for electrochemical properties. As illustrated in Fig. 5(a), the CV curves of N-MCS and MCS at 5 mV s-1 presented a nearly rectangular shape, indicating a favorable

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EDLC behavior combined with pseudocapacitive nature due to the N-doping. According to the CV integrated area, the N-MCS showed higher specific

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capacitance than that of MCS. The pseudocapacitance was observed at the -0.6 V

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[29]. The galvanostatic charge-discharge (GCD) curve with prolonged time at low current density of 0.5 A g -1 (Fig. 5(b)) also confirmed the significantly improved capacitance. The specific capacities calculated by discharge branches were 326 F g 1

for N-MCS, which was more than ca. 2 times higher than MCS (170 F g-1),

indicating that the confined pyrolysis strategy was effective to prepare high electrochemical performance N-MCS. The higher capacitance of N-MCS might be 12

ascribed to its high surface area. The Fig. 5(c) showed the rate performance of NMCS and MCS at the current density range of 0.5-10 A g-1, which demonstrated the capacitance retentions. The specific capacitance of N-MCS decreased to 203 F g-1 at high current density of 10.0 A g-1, with the corresponding capacitance retentions of 62.3 %. The rate capability was shown in Fig. 5(d) when the scan rate increased

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from 5 to 200 mV s-1 and GCD curves of N-MCS at current densities from 0.5 to 10

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A g-1 was shown in Fig. 5(e).

Electrical impedance spectroscopy measurements were furtherly conducted to reveal the capacitive behavior of N-MCS. An equivalent circuit was used to fit the

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impedance spectra as shown in inset of Fig. 5(f), including solution resistance (Rs),

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double layer capacitance (Cdl), charge transfer resistance (Rct), Warburg impedance

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(ZW), leakage capacitance (Q), and the relevant parameters are listed in Table 3. At

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high frequency of the Nyquist plots, Rs could be obtained from the intercept at real

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axis and the N-MCS showed Rs value of 0.34 Ω. The capacitive behavior of N-MCS was further tested in a two-electrode cell

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for practical application. Fig. 6(a) and (b) shows the quasi-rectangular CV curves and triangular GCD curves of N-MCS at working voltage range from 0-0.6 V to 0-

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1.2 V, indicating the maximum working voltage of supercapacitor was no less than 1.2 V, much higher than that of carbon-based symmetric supercapacitors (0.8 or 1.0

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V). Fig. 6(c) presents the rectangular CV profiles of N-MCS at different scan rates, demonstrating N-MCS electrode could serve as promising electrodes for high-rate supercapacitor. Furthermore, its EDLC combined with pseudocapacitive nature was confirmed by the quasilinear GCD curves at different current densities (Fig. 6(d)). The specific capacitance of the N-MCS in two-electrode system was calculated by

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GCD to be 262 F g -1 at a current density of 0.5 A g -1. Moreover, it could be seen that the N-MCS had a relative high capacitance retentions over 64.5% (Fig. 6(e)) when current density increased from 0.5 to 10 A g-1. It was obvious that the capacitance of N-MCS was higher than many other carbon materials, such as solid

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carbon spheres or hollow carbon spheres, etc. as shown in Fig. 6(e) [1, 5, 26, 35-40]. Ragone plots of N-MCS (Fig. 6(f)) were calculated by GCD in the symmetric

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supercapacitor. The energy densities of N-MCS accordingly decreased from 35.8 to 20.5 W h kg-1 when power densities increased from 0.9 to 17.6 kW kg -1. Notably, the N-MCS presented much higher energy density than many carbon spheres with

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different structure and surface property reported previously at corresponding power

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density [41-51]. Another crucial parameter for practical applications of

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supercapacitors was long cycling ability. As displayed in Fig. 7(a), an acceptable

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two-electrode system.

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capacitance retention of 61.1% was obtained for N-MCS after 10000 cyclic tests in

In addition, the GCD curves of N-MCS-EDA, N-MCS-Thiourea and N-MCS-

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Pyridine were also tested to investigate their electrochemical performance as shown in Fig. 7(b) and (c). Compared with the MCS, the N-MCS-EDA, N-MCS-

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Thiourea and N-MCS-Pyridine showed higher specific capacitance as shown in Fig. 7(d), indicating the confined pyrolysis strategy could effectively improve the

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electrochemical performance. The additional pseudocapacitance was also seen at the potential 0.6 V for N-MCS-EDA, N-MCS-Thiourea and N-MCS-Pyridine ascribing to N-doping as shown in Fig. 7(b). All the results indicated that this confined pyrolysis strategy was common and effective to improve electrochemical performance of electrodes by increasing surface area, enlarging pore size and N-

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doping under self-activation effect. 4. Conclusion In summary, we have developed a confined pyrolysis strategy to synthesize Ndoped ordered mesoporous carbon spheres with large mesoporous size, high

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specific surface and pore volume. The synthesis is based on pyrolysis of MPS and nitrogen precursor in a confined nano-space provided by the compact silica shell.

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The released gas (CO2 and H2O etc.) functions as activation agent to enlarge the

pore size and increase the surface area for the carbon materials. Furthermore, the

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nitrogen precursor adsorbed in the pores of the MPS leads to the uniform and

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efficient N-doping in carbon skeleton. The obtained N-MCS possess high surface

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areas, uniform large mesoporous size, high pore volumes and nitrogen content.

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Those excellent characteristics endow the N-MCS with high capacitance and favorable capacitance retention. The confined pyrolysis strategy offers a simple,

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universal and easy way to fabricate functional carbonaceous materials with high surface area, large mesoporous size and doped heteroatoms for a wide range of

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applications.

Acknowledgements

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This work was supported financially by the National Natural Science

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Foundation of China (No. 21676070), the Hebei One Hundred-Excellent Innovative Talent Program (III) (No. SLRC2017034) and the Beijing National Laboratory for Molecular Sciences, Hebei Province Introduction of Foreign Intelligence Projects (2018).

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A

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EP

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CC

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20

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Figure and table captions

Fig. 1. Schematic preparation of N-MCS, MCS and TGA analysis of MPS, MPS/Hmin and Hmim (blue frame) (a), TEM images of MPS@SiO2 core-shell

U

spheres (b, c), SEM images with inset showing EDS analysis (d) and TEM images

N

(e) of N-MCS, TEM images of MCS (f), nitrogen adsorption-desorption isotherms

CC

EP

TE D

M

A

(g) and pore size distribution curves (h) of N-MCS and MCS.

A

Fig. 2. Small angle XRD patterns of MCS and N-MCS (a), XPS patterns (b), C1s (c), O1s (d) and N1s (e) spectrum of MCS and N-MCS, schematic of nitrogen species incorporated into carbon framework (f).

21

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Fig. 3. Schematic illustration for activation and N-doping of N-MCS inside

EP

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M

A

N

U

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compact silica shell.

Fig. 4. TGA analysis of composite of MPS with EDA, Thiourea and Pyridine (a), (b), nitrogen adsorption-desorption isotherms (c), pore size

CC

TEM images

A

distribution curves (d), XPS patterns (e), C1s (f), O1s (g) and N1s (h) spectrum and C, N and O contents (i) of N-MCS-EDA, N-MCS-Thiourea and N-MCS-Pyridine.

22

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Fig. 5. CV (a) and GCD (b) curves at the scan rates of 5 mV s -1 and current density

A

of 0.5 A g-1 respectively; specific capacitances obtained by different GCD (c), CV (d) and GCD (e) curves at different scan rates and current densities of N-MCS, Nyquist plots (f) of N-MCS (Z’ : real part of impedance; Z’’: imaginary part of impedance).

23

IP T SC R U N A M TE D EP CC

A

Fig. 6. CV curves at 50 mV s -1 (a) and GCD curves at 1 A g -1 (b) in different voltage windows for N-MCS, CV curves at different scan rates (c), GCD curves at different current densities (d) of N-MCS, specific capacitance of N-MCS at different current density (e) in two-electrode system and Ragone plots comparison with the reported carbon spheres with different structure of N-MCS (f).

24

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N

Fig. 7. Cycle stability of the electrode at 2.0 A g -1 at two-electrode system of N-

A

MCS (a), CV (b) and GCD (c) curves of MCS, N-MCS-EDA, N-MCS-Thiourea

M

and N-MCS-Pyridine at scan rate of 5 mV s-1 and current density of 1 A g -1 and

A

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capacitance of different samples with different nitrogen precursors (d).

25

Table 1 Textural properties of MCS and N-MCS samples (SBET: BET surface area; Smicro: micropore surface area determined by thet-plot; Vt: total pore volume at P/P0~0.99; Vmicro: micropore volume; Vn=Vmicro/Vt; BJH method was applied to analyze the pore size distribution using the desorption branch of isotherm). Sample

696 1602

493 781

Vt (cm3 g1

) 0.60 2.09

Vmicro (cm3 g-1) 0.23 0.31

Vn (%) 38.3 14.8

A

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M

A

N

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MCS N-MCS

Smicro (m2 g-1)

26

Pore size (nm) 3.0 8.0

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SBET (m2 g-1)

Table 2 Textural properties of N-MCS samples with other nitrogen precursors.

1

1208

) 388

) 4.57

Vmicro (cm3 g-1) 0.07

N-MCS-Thiourea 1288

627

1.31

0.22

16.8

9.4

N-MCS-Pyridine

496

1.26

0.19

15.1

6.7

SBET (m2 g-1)

Sample

892

1

Vt (cm3 g-

1.53

Pore size (nm) 9.4

Vn (%)

A

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M

A

N

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N-MCS-EDA

Smicro (m2 g-

27

Table 3 Parameters of equivalent circuit for N-MCS (CPE: capacitor layer that formed during the charge-discharge process). Rct (Ω)

ZW (Ω)

Cdl (F)

CPE (F)

0.3415

0.0014

1.807

0.0060

0.7264

A

CC

EP

TE D

M

A

N

U

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Rs (Ω)

28